1. Field of the Invention
The present invention relates to an optical scanning device which is used as an optical writing device of a digital image forming device using an electrophotographic method such as a laser printer, a digital copier, a facsimile machine or other such method. The present invention further relates to an optical scanning device and an image forming apparatus including the optical scanning device which has an intensity distribution transforming optical component for transforming the intensity distribution of a light flux, and the image forming apparatus can be used in a digital outputting apparatus, for example, a digital copier, a printer, a facsimile machine or other apparatus.
2. Description of the Related Art
A conventional optical scanning device includes a coupling lens which couples a light flux emitted by a light source to form a parallel light flux, a weakly convergent light flux, or a weakly divergent light flux. An optical deflector deflects the light flux received from the coupling lens at a uniform angular velocity. A scanning image-formation optical system converges the light flux deflected by the optical deflector to form a beam spot on a surface to be scanned, i.e. a photosensitive body, and, thus, the surface to be scanned is scanned with the beam spot. Such an optical scanning device is used as an optical writing device in a digital image forming apparatus using the electrophotographic method such as a laser printer, a digital copier, a facsimile machine or other apparatus.
In such an optical scanning device, in order to achieve high-density writing (more than 1200 dpi, for example), it is necessary to form a beam spot having a sufficiently small diameter.
In order to obtain a beam spot having such a small diameter, it is necessary to increase the NA of the optical system of the optical scanning device. Further, in order to obtain a stable small-diameter beam spot, it is necessary that the optical system provides a large depth of focus which tolerates possible component allowance (the curvature radiuses, thicknesses, refractive indexes) for deviations of optical components of the optical system, mounting errors, and environment variations (temperature, humidity).
Assuming that the intensity distribution on the exit pupil of the optical system is a Gaussian distribution, the allowable degree of depth of focus 2 d is in proportion to the second power of the beam spot diameter w, as shown in the following expression:
2 dxe2x88x9dw2/xcexxe2x80x83xe2x80x83(1) 
In the above expression, xcex represents the used wavelength. Thus, the allowable degree of depth of focus decreases sharply as the beam spot diameter is reduced. Therefore, when reduction of the beam spot diameter is attempted, the allowable degree of depth of focus decreases, and, as a result, it is not possible to obtain a stable small-diameter beam spot when the above mentioned component allowance deviations or environmental variations occur.
One solution to this problem is to generate the zero-order Bessel beam of the first kind and obtain a beam spot having a large allowable degree of depth of focus.
For example, Japanese Laid-Open Patent Application No. 9-304714 discloses an optical system providing a large allowable degree of depth of focus by arranging a shading member having a shading portion which shades a portion of a light flux on an optical path between a light source and an optical deflector.
Further, Japanese Laid-Open Patent Application No. 10-227992 discloses generation of a Bessel beam having an intensity distribution which is approximately in proportion to the second power of the zero-order Bessel function of the first kind, in a system in which a laser beam is incident on a diffraction optical component consisting of a binary optical component having an optical performance approximately equivalent to a conical prism.
However, in each of these systems, the intensity distribution of the beam is axially symmetric. Therefore, when the system is used as an optical system of an optical scanning device, it is difficult to independently set a beam spot in a main scanning direction (in which scanning is performed with a light flux) and in a sub-scanning direction (perpendicular to the main scanning direction).
In the above-mentioned system, a Bessel beam is obtained as a result of transforming the distribution of the amplitude term u1(y1, z1) of the following equation (2) into an arbitrary amplitude distribution. The intensity distribution thereof is expressed by the second power of the amplitude distribution. In the following equation (2), the direction of the optical axis is coincident with the x direction, the main scanning direction perpendicular to the optical axis is coincident with the y direction and the sub-scanning direction also perpendicular to the optical axis is coincident with the z direction.                                           u            2                    ⁡                      (                                          y                2                            ,                              z                2                                      )                          =                                            j              ⁢                              xe2x80x83                            ⁢                              ⅇ                                  -                                      ik                    ⁡                                          (                                              x                        +                                                                                                            y                              2                              2                                                        +                                                          z                              2                              2                                                                                                            2                            ⁢                            x                                                                                              )                                                                                                          λ              ⁢                              xe2x80x83                            ⁢              x                                ⁢                      ∫                          ∫                                                                    u                    1                                    ⁡                                      (                                                                  y                        1                                            ,                                              z                        1                                                              )                                                  ⁢                                  ⅇ                                                            -                      i                                        ⁢                                          k                                              2                        ⁢                        x                                                              ⁢                                          (                                                                                                    y                            1                                                    ⁢                                                      y                            2                                                                          +                                                                              z                            1                                                    ⁢                                                      z                            2                                                                                              )                                                                      ⁢                                  ⅆ                                      y                    1                                                  ⁢                                  ⅆ                                      z                    1                                                                                                          (        2        )            
The above equation (2) is expressed assuming that the intensity distribution u22 (y2, z2) of the beam spot on the image surface is approximately in accordance with the Fraunhofer diffraction.
In the above equation (2):
u2 (y2, z2): the amplitude distribution of the beam spot on the image surface;
u1 (y1, z1): amplitude distribution on the pupil;
xe2x88x92ik (y1y2+z1z2)/2x: phase difference on the pupil (k represents the wave number); and
j/xcex: Fresnel inclination coefficient (where X represents the used wavelength).
The expression of the Fraunhofer diffraction of the above equation (2) has the same meaning as that of Fourier transform expression, and the amplitude distribution u2 (y2, z2) on the image surface is equal to that obtained from Fourier transform being performed on the amplitude distribution u1 (y1, z1) on the pupil. Therefore, the expression of the Fraunhofer diffraction of the above equation (2) is referred to as a Fourier transformed image.
Further, in any method, when a Bessel beam is generated, side lobes develop. Therefore, when the sensitivity of the photosensitive body is high, image degradation such as resolution degradation and/or stain in background occurs.
FIGS. 1, 2A and 2B show an example of an optical scanning device according to the related art. FIG. 1 shows an optical arrangement of the optical scanning device. In FIGS. 2A and 2B, the optical scanning device is shown in a condition in which the optical scanning device is expanded along an optical path of a light flux extending from a light source to a surface to be scanned. FIG. 2A shows the sectional view of the optical scanning device taken along a deflection plane (including the plane formed as a result of the light flux scanning the surface to be scanned), and FIG. 2B shows the sectional view of the optical scanning device taken along the plane including the optical path of the light flux and perpendicular to the deflection plane.
As shown in FIGS. 1, 2A and 2B, the optical scanning device 30 includes a light source 1 which emits a laser light, a first optical system 2 for directing the laser light emitted by the light source 1 to an optical deflecting portion 3, the optical deflecting portion 3 which deflects the light flux from the first optical system 2, and a second optical system 4 for forming a beam spot on the surface 5 to be scanned using the thus-deflected light flux. The above-mentioned first optical system 2 includes a collimating lens 21, an aperture 22 and a cylindrical lens 23. The second optical system 4 includes a spherical lens 41 and an fxcex8 lens 42.
A process of optical scanning will now be described more specifically. The light flux emitted by the semiconductor laser 1, for example, is transformed into an approximately parallel light flux by the collimating lens 21, and passes through the aperture 22. It is also possible to use a coupling lens instead of the collimating lens 21, and to transform the light flux from the semiconductor laser 1 into a weakly divergent light flux or a weakly convergent light flux.
The light flux from the semiconductor laser 1 is transformed into the approximately parallel light flux, which is then converged into a line image elongated in the deflection direction by the cylindrical lens 23, and is directed to the deflection reflective surface of the polygon mirror 3. The light flux deflected by the polygon mirror 3 is incident on the scanning lens 41, and, the beam spot is formed on the surface 5 to be scanned. The characteristics of curvature of field and uniform-velocity characteristics of the scanning lens 41 are well corrected. Further, the light flux deflected by the polygon mirror 3 is first directed to a photodetection portion 6 by an optical-path changing mirror 8 via the scanning lens 41, and is used as a synchronization signal for detecting a position. from which an image is written.
Generally speaking, the intensity distribution of the light flux emitted from a laser light source is a Gaussian distribution. At this time, the depth of focus Z of the light flux which forms a beam spot having a diameter (o on a surface to be scanned is expressed by the following expression;
Z=kxcfx892/xcexxe2x80x83xe2x80x83(a) 
In the above equation (a), k represents a constant, and xcex represents the wavelength.
As can be clearly seen from the above equation (a), as the diameter xcfx89 of the beam spot is reduced, the depth of focus Z decreases at the rate of the second power of the diameter xcfx89 of the beam spot.
The diameter xcfx89 of the beam spot is expressed by the following equation:
xcfx89=Kxcex/NAxe2x80x83xe2x80x83(b) 
In the above equation (b), K represents a constant, xcex represents the wavelength and NA represents the numerical aperture.
Recently, as the resolution of an image outputting apparatus which uses a laser as a light source such as a laser printer is increased and the quality of images obtained therefrom is increased, it is necessary to reduce the diameter of a beam spot on a surface to be scanned, that is, the surface of a photosensitive body in an example of a laser printer.
However, as shown in the equation (a), the depth of focus Z is determined to be in proportion to the second power of the diameter xcfx89 of a beam spot in the case of a Gaussian beam, reduction in the diameter xcfx89 of the beam spot results in decreases in the depth of focus Z, and, thereby, it is difficult to satisfy the allowable range for practical use.
In order to solve the problem, Japanese Laid-Open Patent Application No. 5-307151 discloses an optical scanning device in which a beam spot on a photosensitive body Is formed by a Bessel beam.
The Bessel beam is a non-diffracting beam by using which it is possible to reduce the diameter of a beam spot and to increase the depth of focus in comparison to the above-described Gaussian beam. The Bessel beam has the Intensity distribution approximately in proportion to the second power of the zero-order Bessel function of the first kind. With regard to the Bessel beam, see xe2x80x9cExact Solutions For Non-diffracting Beams"", written by J. Durnin, Vol. 4, No. 4/April 1987/J. Opt. Soc. Am. A, page 651.
Methods of generating the Bessel beam include using a ring-shaped thin slit (see xe2x80x98Diffraction-Free Beamsxe2x80x99, written by J. Durnin et al., Physical Review Letters, Vol. 58, No. 15, Apr. 13, 1987, page 1499), and using an axicon prism (see xe2x80x98Long-Range Laser-Beam Spot Formation By An Axicon Prismxe2x80x99, written by Satoshi Kawata et al., Proceedings of Spring Lecture of Applied Physics Society (1990), page 829) and others.
The above-mentioned Japanese Laid-Open Patent Application No. 5-307151 discloses an optical scanning device which performs image formation on a surface of a photosensitive body using a Bessel beam having an intensity distribution approximately in proportion to the second power of the zero-order Bessel function of the first kind obtained from a laser light.
However, because the Bessel beam develops large side lobes, the image quality is degraded.
In order to reduce the side lobes, Japanese Laid-Open Patent No. 6-148545 discloses an optical scanning device which generates an eccentric Bessel beam, and cuts off the side lobes using a slit member. In this optical scanning device, the slit member having a slit in a direction which coincides with the deflection direction is arranged in proximity to a surface to be scanned.
However, it is difficult to adjust the position of this slit member properly. Further, by using the slit, the quantity of light is greatly reduced. There is another method in which an axicon prism is used for reducing the side lobes. However, in this method using the axicon prism, it is necessary to mount the axicon prism with high accuracy, and. thereby, it is difficult to achieve mass production of optical scanning devices using the axicon prisms.
Japanese Laid-Open Patent Application No. 9-243945 discloses an optical scanning device in which stop means and shading means are provided between a collimating lens and a cylindrical lens. By using the simple means, the diameter of a beam spot is reduced, and the depth of focus is enlarged.
However, also in this method, a portion in the vicinity of the center of a light flux is cut off, and, thereby, the quantity of light is reduced.
Further, recently, consideration of environmental factors is required, and, recycling is being performed for OA equipment such as a copier. Accordingly, designing of structures and components which are suitable for recycling is becoming advanced, and, also, designing of components which can be used in common for various devices is being further developed.
To overcome the problems described above, preferred embodiments of the present invention provide an optical scanning device including an optical system which provides a large degree of depth of focus in order to obtain an excellent small-diameter beam spot even when component allowance (the curvature radiuses, thickness, refractive indexes) deviations of optical components of the optical system, mounting errors, environment variations (temperature, humidity), or other such problems occur.
Furthermore, preferred embodiments of the present invention provide an optical scanning device including an optical system through which the side lobes of the Bessel beam are minimized.
Another preferred embodiment of the present invention provides an optical scanning device which minimizes the diameter of a beam spot in a simple manner and without any adverse affects due to the side lobes and without causing reduction of the quantity of light.
Another preferred embodiment of the present invention provides an optical scanning device which is produced using components which can be used in common in various types of devices, and/or by recycling of a device in the related art.
In one specific preferred embodiment of the present invention, an optical scanning device includes a coupling lens which couples a light flux from a light source to form a parallel light flux, a convergent light flux or a divergent light flux, an optical deflector including a deflective reflection surface which deflects the light flux from the coupling lens at a uniform angular velocity, a scanning image-formation optical system which converges the deflected light flux from the optical deflector on a surface to be scanned to form a beam spot, wherein the device scans the surface to be scanned with the beam spot at a uniform velocity and the device further includes a depth increasing component, for increasing the depth of focus determined by the entire optical system of the device, on the optical path between the light source and the deflective reflection surface.
As a result of this novel combination of structural elements, it is possible to provide an optical scanning device which achieves a large allowable degree of depth of focus determined by the entire optical system of the device.
Accordingly, it is possible for the optical scanning device providing a large allowable degree of depth of focus to produce excellent small-diameter beam spots even when component allowance (the curvature radiuses, thickness, deviations of optical elements of mounting errors, and environment refractive Indexes) the optical system, variations (temperature, humidity) occur.
The depth increasing component may preferably include an intensity-distribution transforming component which transforms the intensity distribution of the light flux such that the intensity distribution on the exit pupil is changed so that the intensity at four corners thereof is higher than that at a central portion.
As a result of this unique structure, the optical scanning device achieves a large allowable degree of depth of focus determined by the entire optical system of the device.
The depth increasing component may also include a phase-distribution transforming component which transforms the phase distribution of the light flux such that the phase distribution on the exit pupil is arbitrarily changed.
As a result, the optical scanning device achieves a large allowable degree of depth of focus determined by the entire optical system of the device.
The depth increasing component may also include an intensity-distribution transforming component which transforms the intensity distribution of the light flux so that the intensity distribution on the exit pupil is changed, and a phase-distribution transforming component which transforms the phase distribution of the light flux such that the phase distribution on the exit pupil is changed.
As a result, the optical scanning device achieves a large allowable degree of depth of focus determined by the entire optical system of the device, and small side lobes.
The depth increasing component may also include an optical component obtained as a result of integrating an intensity-distribution transforming component which transforms the intensity distribution of the light flux such that the intensity distribution on the exit pupil is changed, and a phase-distribution transforming component which transforms the phase distribution of the light flux such that the phase distribution on the exit pupil is changed.
Thereby, the optical scanning device achieves a large allowable degree of depth of focus determined by the entire optical system of the device, and small side lobes, in a very compact configuration.
In another preferred embodiment of the present invention, an optical scanning device includes a light source which emits a light flux, an optical deflector which deflects the light flux, a first optical system which directs the light flux emitted from the light source to the optical deflector, and a second optical system which directs the light flux deflected by the optical deflector onto a surface to be scanned, wherein the first optical system includes an intensity-distribution transforming unit which has at least one intensity-distribution transforming lens and transforms the intensity distribution of the light flux emitted from the light source into an arbitrary intensity distribution.
As a result of this unique structure and arrangement thereof, the optical scanning device simply and reliably produces beam spots having a very small diameter, increases the depth of focus determined by the entire optical system of the device, and, at the same time, decreases the side lobes of the beam spot, without decreasing the quantity of light. Further, it is possible to use components of optical scanning devices in common and to recycle the optical scanning devices.
The first optical system may further include an optical coupling component which couples the light flux emitted from the light source and an optical converging component which converges, at least in the direction perpendicular to the deflection direction, the light flux in proximity to the optical deflector.
As a result of this structure which is relatively simple, i.e., the two optical components and the intensity-distribution transforming unit, the optical scanning device produces beam spots with minimal diameter, increases the depth of focus determined by the entire optical system of the device, and, at the same time, decreases the side lobes of the beams spot, without decreasing the quantity of light. Further, it is possible to use components of optical scanning devices in common and to recycle the optical scanning devices.
The first optical system may further include a collimating lens which transforms the light flux emitted from the light source into an approximately parallel light flux, and a cylindrical lens which has power in the sub-scanning direction and the intensity-distribution transforming unit may transform the intensity distribution of the light flux and emits the resulting light flux in a form of an approximately parallel light flux.
As a result, through a simple structure, i.e., the general-purpose collimating lens and cylindrical lens, and the intensity-distribution transforming unit, the designing of which for transforming an approximately parallel light flux into an approximately parallel light flux is relatively easy, the optical scanning device produces beam spots having a much smaller diameter, increases the depth of focus determined by the entire optical system of the device, and, at the same time, decreases the side lobes of the beam spot, without decreasing in the quantity of light. Further, it is possible to use components of optical scanning devices in common and to recycle the optical scanning devices.
The at least one intensity-distribution transforming lens may also include an axially symmetric aspherical lens.
Thereby, it is possible to miniaturize the device, and, because working of the axially symmetric aspherical lens is relatively easy, it is possible to reduce the cost of the device.
The intensity-distribution transforming unit may also include an axially symmetric aspherical lens in a form of a single lens.
Thereby, it is possible to miniaturize the device, and, because working the axially symmetric aspherical lens in a form of a single lens is relatively easy, it is possible to reduce the cost of the device.
The at least one intensity-distribution transforming lens may include a special toric lens.
Thereby, it is possible to change the intensity distribution of the light flux arbitrarily, and it is possible to increase the degree of freedom in the intensity-distribution transformation.
The intensity-distribution transforming unit may include a special toric lens in a form of a single lens.
Thereby, it is possible to change the intensity distribution of the light flux arbitrarily, and it is possible to increase the degree of freedom in the intensity-distribution transformation.
The light source may include a semiconductor laser which emits a light flux having an approximately Gaussian distribution and the light flux directed onto the surface to be scanned may have an intensity distribution which is a non-Gaussian distribution.
Thereby, it is possible to provide a less-expensive, small-sized optical scanning device.
The diameter of the beam spot of the light flux directed onto the surface to be scanned may be smaller than the diameter of the beam spot obtained when the intensity distribution of the light flux is not changed.
Thereby, through a simple arrangement, it is possible to obtain the high-resolution optical scanning device. Further, it is possible to obtain a small beam spot easily by combining the intensity-distribution transforming unit into the optical scanning device in the related art.
The intensity of the highest side lobe of the light flux directed onto the surface to be scanned may be smaller than 1/e2 of the peak intensity of the light flux.
Thereby, as a result of increasing the depth of focus determined by the entire optical system of the device and also reducing the intensity of the side lobes of the light flux through a very simple structure, the optical scanning device forms very high-quality images. Further, by combining the intensity-distribution transforming unit into the optical scanning device in the related art, it is possible to increase the depth of focus determined by the entire optical system of the device, and, also, at the same time, reduce the intensity of the side lobes of the light flux easily.
Further, by using any of the above-described optical scanning devices as an exposing unit of an image forming apparatus, it is possible to minimize the diameter of the beam spot, and, by increasing the depth of focus determined by the entire optical system of the device, and, also, at the same time, reducing the intensity of the side lobes of the light flux, it is possible to obtain high-resolution, high-quality images.
Other advantages, features, characteristics and elements of the present invention will become more apparent from the following detailed description of preferred embodiments when read in conjunction with the accompanying drawings.